Conventionally, there has been known a device which amplifies a signal by means of a variable-gain amplifier, outputs an amplified signal, detects the output by peak detection, processes the detected signal by means of an analog circuit (refer to FIG. 5 of Japanese Laid-Open Patent Publication (Kokai) No. H11-154839) or a digital circuit (refer to FIG. 1 and ABSTRACT of Japanese Laid-Open Patent Publication (Kokai) No. H11-154839), and controls the gain of the variable-gain amplifier based on a processing result.
However, according to the above described prior art, if the output signal of the variable-gain amplifier contains signals of multiple frequency components, it is not possible to maintain a constant level of a signal of a certain frequency component. This is because signals of all the frequency components are detected, when the output signal is detected by the peak detection.
It is therefore an object of the present invention to maintain a constant level of a signal of a certain frequency component of the output signal of the variable-gain amplifier.